Re: [PATCH v10 12/12] drivers/block/pmem: Map NVDIMM with ioremap_wt()

[H. Peter Anvin]

Nontemporal stores to WB memory is fine in such a way that it doesn't pollute the cache.  This can be done by denoting to WC or by forcing cache allocation out of only a subset of the cache.

On May 29, 2015 2:46:19 PM PDT, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>On Fri, May 29, 2015 at 2:29 PM, Elliott, Robert (Server Storage)
><Elliott@xxxxxx> wrote:
>>> -----Original Message-----
>>> From: Andy Lutomirski [mailto:luto@xxxxxxxxxxxxxx]
>>> Sent: Friday, May 29, 2015 1:35 PM
>> ...
>>> Whoa, there!  Why would we use non-temporal stores to WB memory to
>>> access persistent memory?  I can see two reasons not to:
>>
>> Data written to a block storage device (here, the NVDIMM) is unlikely
>> to be read or written again any time soon.  It's not like the code
>> and data that a program has in memory, where there might be a loop
>> accessing the location every CPU clock; it's storage I/O to
>> historically very slow (relative to the CPU clock speed) devices.
>> The source buffer for that data might be frequently accessed,
>> but not the NVDIMM storage itself.
>>
>> Non-temporal stores avoid wasting cache space on these "one-time"
>> accesses.  The same applies for reads and non-temporal loads.
>> Keep the CPU data cache lines free for the application.
>>
>> DAX and mmap() do change that; the application is now free to
>> store frequently accessed data structures directly in persistent
>> memory.  But, that's not available if btt is used, and
>> application loads and stores won't go through the memcpy()
>> calls inside pmem anyway.  The non-temporal instructions are
>> cache coherent, so data integrity won't get confused by them
>> if I/O going through pmem's block storage APIs happens
>> to overlap with the application's mmap() regions.
>>
>
>You answered the wrong question. :)  I understand the point of the
>non-temporal stores -- I don't understand the point of using
>non-temporal stores to *WB memory*.  I think we should be okay with
>having the kernel mapping use WT instead.
>
>--Andy

-- 
Sent from my mobile phone.  Please pardon brevity and lack of formatting.
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